Dynamic modeling and feedback control for conveyors-belt dryers of mate leaves

A semi-empirical model is proposed to reproduce the kinetics of drying in a continuous shallow packed bed dryer of mate leaves at transient conditions. The mathematical expression representing the dynamic model, which was obtained from a mass balance for water in the solid phase of the drying chamber, was validated at steady-state condition in an industrial continuous dryer. The transient model was solved with the numerical method of lines by involving a backward differentiation formula (BDF) to approximate the first order spatial and time derivative. Based upon this reliable model, a control strategy was suggested to maintain the discharge moisture content in the acceptable range of 2.4–3.4% (dry basis) by adjusting the velocity of conveyor-belt to compensate disturbances in the operating conditions. The performance of a proportional-integral-derivative (PID) and a proportional-integral (PI) controller was verified by a comparison between open- and closed-loop responses of discharge moisture content to random changes in the feed moisture content, drying temperature and air velocity. The simplex method was applied during the tuning procedure of the controller parameters by minimizing the integral squared error (ISE) of the process output when a step change in the set-point was imposed.